Particle concentration measurement technology

ABSTRACT

A method and apparatus for measuring particle concentration and size distribution of particles in liquids. The method involve separating dissolved and particulate residues in liquids for determination of the size and concentration of the particulate species. The method includes the steps of forming an aerosol from the liquid sample to be analyzed, evaporating the droplets in the aerosol to dryness, and detecting the particles. An apparatus for separating dissolved and particulate residues in liquids for determination of the size and concentration of the particulate species is also disclosed. The apparatus includes a droplet former, a dryer communicatively connected to the droplet former, and a detector communicatively connected to the evaporator for detecting particles.

CROSS-REFERENCE TO RELATED APPLICATIONS, IF ANY

This application claims the benefit under 35 U.S.C. §119(e) ofco-pending U.S. Provisional Patent Application Ser. No. 61/011,901,filed Jan. 22, 2008, which is hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX, IF ANY

Not applicable.

BACKGROUND

1. Field

The present invention relates, generally, to analysis methods andapparatus for use with compositions of matter. More particularly, theinvention relates to a method and apparatus for measuring the size andconcentration of small particles in high purity liquids and colloidalsuspensions. Most particularly, the invention relates to an apparatusand method for separating dissolved and particulate residues in a liquidto determine the size distribution and concentration of the particulatespecies (i.e. particles). The technology is useful, for example, foraccurate measurement of low concentrations of very small (sub 50 nm)particles in high purity liquids, the measurement of particle retentionby filters, and measurement of particle size distributions in colloidalsuspensions. The invention is suitable for use in the semiconductordevice manufacturing industry, the ink manufacturing industry, and inother fields.

2. Background Information

The present invention has utility in measurement of the concentration ofsmall, for example, sub 50 nm., particles in high purity liquids. Smallparticles are a major problem for the semiconductor device manufacturingindustry. Particles smaller than 50 nm can significantly reducemanufacturing yield of present day semiconductor devices. The ability tomeasure concentrations, especially low concentrations, of theseparticles is highly desired. Insofar as is known, there is no technologyto meet this need.

The present invention also has utility in measurement of particleretention by filters, particularly those with pore sizes smaller thanabout 50 nm. Microporous membrane filters are often used to reduceparticle levels in liquids for semiconductor device manufacturing. Theability of filters of this type to remove particles from the liquids isusually determined by challenging the filters with particles andmeasuring what comes through. Instruments capable of measuring particlesof these sizes are not believed to be available.

The invention further has utility in measurement of Particle SizeDistributions (PSD) in colloidal suspensions. There are numerousapplications in which the size distribution of particles in colloidalsuspensions is important in determining the efficacy of the suspension.Examples include slurries used in chemical mechanical planarization(CMP) of silicon wafers, as well as wafers composed of other materials,during semiconductor chip manufacturing and pigment-based inks. The PSDof CMP slurries determines the planarization rate, surface smoothnessand scratch density on the wafer surface following the CMP process. Allof these are important in determining the finished semiconductor deviceyield and performance. The size distribution of pigment inks isimportant in determining color development.

Historically, the first application mentioned above, measurement ofconcentrations of small particles (particularly those less than 50 nm insize) in high purity liquids has been addressed using single particleoptical particle counters (OPCs). These instruments size and countindividual particles as they pass through a laser beam. They have metthe need of the semiconductor industry until recently, although theyhave typically been believed to have been a half step behind indevelopment. Approximately twenty years ago the industry needed todetect roughly 500 nm particles; now they desire to measure 20-30 nmparticles. The problem is that below about 300 nm the amount of lightscattered by a particle is proportional to the 6^(th) power of thediameter (D_(P) ⁶). Therefore, an instrument to measure 30 nm particlesneeds to be 1,000,000 times more sensitive than one that measures 300 nmparticles. A leading company in making these counters has been ParticleMeasuring Systems. Their highest sensitivity in water is claimed to be50 nm, but it is believed to be closer to 60-70 nm. Claimed sensitivityin chemicals is 65 nm. Particle Measurement Systems had a counter with aclaimed sensitivity of 30 nm counter on the market at one time, but itis no longer available. Other companies that make counters of this typeare RION, Horiba, Particle Sizing Systems, and Hach Ultra.

Very small particles (typically smaller than 10 nm) in liquids have alsobeen analyzed using a combination of electrospray and mass spectroscopy.Electrospray is used to generate small droplets by subjecting the liquidto a high electric field. The liquid must be moderately conductive andthe droplets become highly charged during formation. High purity liquidstypically have low conductivity making the formation of small dropletsdifficult. Also, the high charge on the particles can result in particleagglomeration and may cause other changes in particle properties. Theagglomeration issue can be addressed by exposing the aerosol to ionizingradiation.

The second application, measurement of particle retention by filterswith small pore sizes, has also been addressed using OPCs, again limitedto 50 nm. Other techniques have also been used such as turbidimitry.However, these techniques can only be used at very high particleconcentrations, concentrations well above those seen in high purityapplications. And, filter performance at these high concentrations isnot representative of performance at lower concentrations.

Filter performance has also been measured using non-volatile residuemonitors (NVRM or NRM). These instruments work by forming an aerosol ofthe particle-laden liquid, evaporating the liquid in the aerosol andmeasuring the number of particles in the aerosol. The problem with thismethod is that any dissolved material in the liquid forms a particlewhen the liquid is evaporated. Hence, the instrument measures bothdissolved and particulate residue. And the residue particles interferewith the particulate particle measurement.

The third application, measurement of particle size distributions incolloidal suspensions, has typically been addressed using either dynamiclight scattering (DLS) or centrifugal sedimentation. Both of thesemethods only measure relative PSDs. They cannot determineconcentrations.

For these and other reasons, a need exists for the present invention.

All US patents and patent applications, and all other publisheddocuments mentioned anywhere in this application are hereby incorporatedby reference in their entirety.

BRIEF SUMMARY

The present invention provides a method and apparatus for measuring (a)the concentrations of small particles, on the order of 50 nm or smaller,and (b) the size distributions of such particles, in liquids, whichmethod and apparatus are practical, reliable, accurate and efficient,and which are believed to fulfill a need and to constitute animprovement over the background technology.

In a basic embodiment, the method of the present invention includes thesteps of, providing a specimen to be tested, isolating small, uniformlysized droplets from the specimen, evaporating the droplets to dryness,and counting and sizing the particles that were originally (initially)in the liquid. Thereby, particle concentration and PSD may bedetermined. The method is especially effective for measuring lowconcentrations of very small particles, particularly those less than 50nm in size.

In one aspect of the present invention, an apparatus includes aNebulizer/Impactor and a condensation particle counter (CPC). TheNebulizer/Impactor has means to form or isolate small, uniformly sizeddroplets. The CPC accurately counts particles present after the small,uniformly sized droplets are dried. This embodiment is believed to bebest suited for high purity liquid to measure particle concentrationabove a defined threshold, but without PSD measurement, a ThresholdParticle Counter (TPC).

In another aspect, the apparatus includes a Nebulizer/Impactor and aScanning Mobility Particle Sizers (SMPS), which performs both particlecounting and sizing. This embodiment is believed to be best suited fordetermining PSD in addition to particle concentration.

In a further aspect, a Nebulizer/Impactor combination is provided forgenerating an aerosol composed of multiple droplets of a liquid. TheNebulizer-Impactor includes a housing forming a mixing chamber having(i) a liquid entrance for receiving a sample liquid into the chamber,(ii) a primary orifice having a first diameter for receiving apressurized gas into the chamber for merger with the sample liquid togenerate an aerosol composed of multiple droplets of the sample liquidsuspended in the gas, and (iii) a secondary orifice having a secondarydiameter for conducting the aerosol out of the chamber. The secondorifice is less than a major dimension of the mixing chamber taken in adirection substantially perpendicular to an axis of the secondaryorifice, so as to restrict flow out of the mixing chamber to generate aback pressure in opposition to entry of the sample liquid and thepressurized gas into the chamber.

In contrast to other nebulizers in which the chamber exit is simply opento the downstream components with a diameter equal to that of thechamber, the exit orifice in the nebulizer in D has a diameter less thanthat of the chamber, more preferably less than half the diameterchamber. The diameter reduction provides a constriction that produces ahigher kinetic energy mixing of the gas and liquid in the merger zone.As a result, the nebulizer generates smaller droplets. The secondaryorifice also helps direct the aerosol toward the impactor surfaceraising the impactor efficiency.

Another factor reducing the droplet size produced by theatomizer/impactor is close axial positioning of an impactor justdownstream of the secondary orifice. The more closely spaced impactorremoves a greater proportion of the larger droplets.

In a preferred version of nebulizer/impactor, the impactor axial spacingfrom the secondary orifice is adjustable through movement of theimpactor. For example, a threaded mounting of the impactor to thenebulizer frame allows axial position adjustment by turning the impactorabout its longitudinal axis. The average size of the droplets in theaerosol leaving the nebulizer can be increased or decreased byrespectively enlarging or reducing the axial spacing between thesecondary orifice and the impactor. The average size can also bedecreased and the uniformity increased by making the shape of thehousing containing the secondary orifice conformal to the impactorshape.

The droplet size produced by atomizer/impactor also can be adjusted bychanging or selecting the secondary orifice. Reducing the diameter ofthe secondary orifice is believed to increase back pressure and reducedroplet size. It has been found useful to provide a secondary orificewith a diameter larger than that of the primary orifice. The ratio ofthe secondary orifice diameter to the primary orifice diameter can rangefrom slightly above one, to about two in versions that incorporate asecondary orifice.

The aspects, features, advantages, benefits and objects of the inventionwill become clear to those skilled in the art by reference to thefollowing description, claims and drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The present invention, and the manner and process of making and usingit, will be better understood by those skilled in the art by referenceto the following drawings.

FIG. 1 is a flow diagram of one embodiment of the method of the presentinvention.

FIG. 2A is a diagram illustrating an embodiment of the apparatus of thepresent invention.

FIG. 2B is a diagram illustrating an alternative embodiment of theapparatus of the invention.

FIG. 2C illustrates an embodiment of an apparatus for testing orcharacterizing optimized threshold particle counting.

FIG. 2D illustrates a system for measuring droplet size distributionsproduced by droplet formers.

FIGS. 3A-C illustrate pneumatic, concentric and flow focusingembodiments of a nebulizer component of the apparatus of the invention.

FIG. 4 is a crossectional view of an embodiment of a plate impactorcomponent which is used in an embodiment of the apparatus of theinvention.

FIG. 5 is a crossectional view of an embodiment of a virtual impactorwhich is used in another embodiment of the apparatus of the invention.

FIG. 6 is a diagram showing impactor efficiency.

FIG. 7 illustrates an embodiment of a vibrating orifice generator usedin an embodiment of the apparatus of the invention.

FIG. 8 is a sectional side elevation view of an embodiment of the systemof the present invention including a combination nebulizer-impactor.

FIG. 9 is a sectional view of the combination nebulizer-impactor.

FIG. 10 is an enlarged view showing a portion of the nebulizer-impactorof FIGS. 8 and 9.

FIG. 11 illustrates an embodiment of a condensation particle counterused in an embodiment of the apparatus of the invention.

FIG. 12 is a graph of droplet size distributions produced by variouscombinations of nebulizers with impactors.

FIG. 13 is a graph of the cumulative particle concentration over timeusing a nebulizer-impactor Combination D.

FIG. 14 is a graph of particle cumulative concentration versus time fordetection of particles in UPW, with respect to 30 nm polystyrene latex(PSL) particles.

FIG. 15 is a graph of particle cumulative concentration versus time fordetection of particles in ultra pure water (UPW) with respect to 22 nmsilica particles.

FIG. 16 is a graph of differential concentration versus particle size,which shows the ability to size 30 nm particle PSL.

FIG. 17 is a graph of CMP slurry PSD measured using a Combination Dapparatus with an SMPS detector.

FIG. 18 compares an embodiment of the method of the invention with DLS.

FIG. 19 shows the change in slurry PSD over time during handling via agraph of differential number concentration versus particle diameter.

FIG. 20 shows particle concentrations of particles upstream (feed) anddownstream (filtrate) of a test filter.

FIG. 21 is a graph of percentage retention of a filter versus particlediameter, which demonstrates retention of the filter as a function ofparticle size.

DETAILED DESCRIPTION

The present invention provides a method and apparatus for determiningthe size distribution and concentration of particles in a liquid.

A. Methods of the Invention.

The method involves (a) forming droplets, for example viaaerosolization, from a liquid sample to be analyzed, (b) isolating smalldroplets from the droplets, for example less than 10 um in size, (c)drying the droplets to remove the liquid, for example via evaporation,and (d) counting the residual particles.

Importantly, the aerosol droplets isolated are small and uniformlysized, less than 10 um and preferably a median size less than 1 um. Thedroplets must be small and uniformly sized because any dissolvedmaterial in the droplet will form a “residue” particle as a result ofdrying. If the residue particle is large enough it will be detected bythe analyzer and interfere with the measurement of true, non-dissolved,particles.

The size of a residue particle resulting from evaporation of a liquiddroplet can be determined from the concentration of the non-volatileresidue in the droplet using equation 1 where d_(s) is the size of thefinal residue particle, d_(d) is the size of the droplet diameter andF_(v) is the volume fraction of non-volatile residue in the droplet:

d _(s) =d _(d)(F _(v))^(1/3)   (1)

If the density of the non-volatile residue in the droplet is the same asthe liquid (1.0 g/cm³ in the case of water), then F_(v) is simply theweight concentration of non-volatile residue (C). If the water has anon-volatile concentration of 1 ppb with a density of 1.0, equation 1above can be used to calculate the minimum droplet size that will yielda 25 nm particle, as follows:

d _(d) =d _(s)(C)^(−1/3)=25 nm (10⁻⁹)^(−1/3)=25 nm (1000)=25 μm

Hence, the residue in droplets smaller than 25 μm will not be detectedby a 25 nm CPC, if the CPC has a very sharp size cutoff.

The required small, uniformly sized droplets required by the presentinvention may be generated by firstly making droplets of diverse sizesand secondly removing large droplets. Alternatively, the desireddroplets may be made in a single step. An example of the formerembodiment of the method is implemented by generating droplets by acompressed air nebulizer or an ultrasonic nebulizer and then removinglarge droplets by directing them to an impaction surface such as a plateimpactor or a virtual impactor. An example of generating small, uniformdroplets directly is by way of a vibrating orifice aerosol generator.After the droplets are formed, liquid in the droplets is removed beforethe droplets collide or coalesce. Liquid removal may be accomplished byheating to dry via dilution air, heated air, or heating the liquid. Itmay also be accomplished by evaporation. And after drying to isolate theparticles, the particles are counted and/or sized by OPC, CPC, SMPS orother instruments.

Thus, referring to FIG. 1, a flow chart of a basic embodiment of themethod 10 of the invention involves the steps of, providing 11 a liquidsample, forming 12 very small, uniformly sized droplets, via an aerosol,from the liquid sample to be analyzed, drying 13, via evaporating, thedroplets in the aerosol, and counting 14 the residual particles.Variants of this embodiment of the method are discussed above.

B. Apparatus of the Invention.

Referring to FIG. 2A, one embodiment of the apparatus 20 of the presentinvention comprises means 22 for forming droplets of diverse sizesconnected to a sample input 21. Means 23 for removing large droplets iscommunicatively connected to the means 22 for forming droplets Anexample of droplet former 22 is a compressed air nebulizer, anultrasonic nebulizer, or a flow-focusing nebulizer examples of which areshown in FIG. 3. An example of means 23 for isolating small uniformlysized droplets is a plate impactor or a virtual impactor (examples ofwhich are shown in FIGS. 4 and 5) having an impaction surface at whichan aerosol stream output by the nebulizer 22 is directed and whichremoves large droplets. After the desired droplets are formed, liquid inthe droplets is removed before the droplets collide or coalesce byliquid removal means 24. Examples of such means include a stream ofdilution air, heated air, or a liquid heater. Drying may also beaccomplished by a fast evaporator. A particle analyzer 25 iscommunicatively connected to the liquid remover 24. Examples of suchanalyzer for counting and/or sizing includes an OPC, CPC, SMPS or otherinstrument or combination of instruments. Exemplary nebulizers,impactors and analyzers are described in detail below.

Referring to FIG. 2B, another embodiment of the apparatus 30 of thepresent invention comprises means 32 for making droplets of a small anduniform size connected to a sample input 31. An example of such means isa vibrating orifice aerosol generator, an example of which is describedin detail below. After the desired droplets are formed, liquid in thedroplets is removed before the droplets collide or coalesce by liquidremoval means 33. Examples of such means include a stream of dilutionair, heated air, or a liquid heater. Drying may also be accomplished bya fast evaporator. A particle analyzer 34 is communicatively connectedto the liquid remover 33. Examples of such analyzer for counting and/orsizing includes an OPC, CPC, SMPS or other instrument or combination ofinstruments.

Testing of threshold particle counting of embodiments of the apparatusof the invention may be performed using the system 100 shown in FIG. 2C.In this system 100, particles of various types and sizes, for example ahigh purity liquid or a colloidal suspension, are injected into aflowing stream of UPW (input 110). Concentrations of the injectedparticles are measured using an HSLIS-M50 optical particle counter (OPC)120 (of Particle Measuring Systems). A portion of the water is sentthrough a TPC combination nebulizer/impactor 130. The resulting aerosolis analyzed using 3 detector embodiments:

-   -   A 20 nm CPC 140    -   An SMPS 150, for example capable of measuring aerosol particle        size distributions from 5 to 1000 nm.    -   A dew point sensor 160

The dew point sensor 160 is preferably used to measure the water vaporcontent of the gas containing the nebulized liquid to determine theinstrument inspection volume. Under present optimum conditions, theinstrument inspection volume is as large as practicable, preferablyapproximately 50-10,000 μlit/min [>100 μlit/min].

Processing may be performed with monodisperse polystyrene latex (PSL)particles of various sizes, polydisperse PSL, monodisperse silicaparticles or other particle types. For example, measurements may be madewith 30 nm PSL, polydisperse PSL and 22 nm silica. The size distributionof the monodisperse particles tested can also be measured using a NICOMP380ZLS (of Particle Sizing Systems). This instrument measures sizedistributions via dynamic light scattering (DLS)—a commonly usedtechnique for measuring the size of small colloidal particles and thetechnique used by Duke Scientific (PSL bead supplier) to measure sub-50nm PSL.

An apparatus for measuring the size distribution of droplets formed byvarious droplet forming methods is shown in FIG. 2D. A test solution 51is input to a vessel 52. Solution 51 is output via a gear pump 53through a filter 54 and into a small overflow vessel 56. Most of theliquid input to vessel 56 returns to vessel 52. A small portion of theliquid is sent to droplet former 55. Droplet former 55 forms an aerosol57 containing small droplets. The nebulizer 55 is connected to apressurized gas source 58, preferably compressed air or N₂. The gas isfiltered, for example via a Wafergard filter 59. Various droplet former55 embodiments are discussed below.

The aerosol 57 is input by the droplet former 55 to a drying chamber 70.The drying chamber 70 is an elongated structure with input and outputends, a predetermined length and a predetermined horizontal dimension.The drying chamber 70 input end is connected to a source of room air viaa pump 71. Air is preferably filtered, for example via a Millipore 0.22micron Hydrophobic Millipak filter 72. The droplet former 55 is disposedat a predetermined location on the drying chamber 55. A ScanningMobility Particle Sizer (SMPS) 33 is disposed at a predeterminedlocation on the drying chamber 55 a predetermined distance “L” from thenebulizer 55. A vacuum pump 74 is connected to the SMPS 33. The pump 74operates at about 1.54 liters per minute. A thermohygrometer 75, forexample a DigiSense meter is disposed at the output end of the dryingchamber 70, a predetermined distance “l′” further downstream from theSMPS 33.

Referring to FIG. 3A, an example pneumatic nebulizer 200 which may beused to create initial droplets is disclosed. In a typical pneumaticnebulizer 200 with vent 206, compressed air exits from a small orifice201 at high velocity creating a low pressure in the exit region. The lowpressure causes liquid to be drawn into the airstream from a second tube203 from liquid reservoir 202. The high velocity air causes the liquidto accelerate and break into droplets. The high velocity spray 204 isdirected toward an impaction surface where the largest droplets areremoved and an aerosol 205 is output.

Commercially available nebulizers typically generate aerosols withdroplets whose size is log-normally distributed. Median droplet sizesare typically 0.5-5.0 μm. The geometric standard deviation is typically˜2.0. The large geometric standard deviation means that the nebulizersgenerate a significant number of large droplets. For example,approximately 0.0003% of the droplets from a nebulizer producing anaerosol with a median droplet size of 1.0 pm and geometric standarddeviation of 2.0 would be larger than 25 μm. This is in an unacceptablenumber of large droplets for the applications described above. Examplesof commercially available pneumatic nebulizers include Laskin nebulizer,Babington nebulizer, Cross-flow nebulizer, and Pre-filming nebulizer.Referring to FIGS. 3B and 3C, known concentric 210 and flow focusingpneumatic 220 nebulizers might also be used. Ultrasonic generators arealso useable for generating small droplets, but less preferred thannebulizers.

As was discussed above, large droplets can be removed from the aerosolusing either a plate impactor 300, shown in FIG. 4, or a virtualimpactor 350 as shown in FIG. 5. In the plate impactor, the platedeflects the aerosol flow to follow an abrupt 90° bend. Droplets withsufficient inertia deviate from the flow stream, impact on the plate,and are removed from the gas stream. A virtual impactor is similar to aplate impactor except that the droplets are impacted into a quiescentregion where they are withdrawn from the aerosol by a small secondaryflow.

The effectiveness of impactors for removing particles is related to theStokes number or impaction parameter. The Stokes number (S_(tk)) isproportional to the square of the droplet size as shown in equation 2where ρ_(p) is the droplet density, U is the nozzle velocity, η is thegas viscosity, and D_(j) is the nozzle diameter.

S _(tk)=(ρ_(p) d _(p) ² U)/(9ηD _(j))  (2)

Impactors can be designed with sharp efficiency curves. An impactordesigned to remove 50% of the droplets >10 μm should remove virtuallyall droplets >25 μm. An example of a typical impactor efficiency curveis shown in FIG. 6.

Another approach to generating an aerosol with small droplets is throughthe use of a vibrating orifice aerosol generator 400. Referring to FIG.7, these generators 400 work by vibrating a liquid at a high frequencyas it passes through a small orifice. They produce nearly monodispersedroplets. The size of the droplets generated can be calculated usingequation 3 where Q_(L) is the liquid flow rate and f is the oscillatingfrequency:

d _(d)=(6Q _(L) /πf)^(1/3)  (3)

A generator operating at 2 MHz with a flow rate of 0.02 ml/min wouldproduce 10 μm droplets.

A preferred approach involves a system including a combinationnebulizer-impactor 450. Referring to FIGS. 8-10, an input conduit 428transfers fluid to a pneumatic nebulizer portion of the system. Thenebulizer also receives air, nitrogen or another gas under pressure froma pressurized gas source through conduit 460. Within nebulizer 450, theliquid sample and compressed gas are merged to generate an aerosolincluding droplets of the liquid sample suspended in the gas.

Nebulizer 450 includes a reservoir 468 in fluid communication with themerger zone. The reservoir collects most of the liquid supplied throughthe input conduit 428, i.e. the liquid not used to form the aerosoldroplets.

The inclined orientation shown is advantageous for liquid drainage andevacuation, although not critical. A housing of the nebulizer hasseveral integrally coupled sections, including a stainless steel housingsection 472 that encloses merger zone 448, a steel housing section 474forming the aerosol conditioning zone, and a housing section 476providing the reservoir. Housing section 472 supports a fitting 478 forreceiving the air or other compressed gas from conduit 460. This housingsection also supports an impactor 480, through a threaded engagementthat permits adjustment of the axial spacing between impactor 480 andmerger zone 448.

With reference to FIG. 9, housing section 472 further supports athermoelectric device 482 that functions to maintain a stabletemperature of about 30.degree. C. in the vicinity of merger zone 448.More particularly, the thermoelectric device extracts heat from housingsection 472 and transfers it to a heat sink 484. The thermoelectricdevice also may function as a heater for the nebulizer. The constanttemperature promotes consistent droplet formation. Housing section 472further supports bulkhead fitting 446, which secures an input conduit428 used to transfer the sample liquid to merger zone 448.

As best seen in FIG. 10, merger zone 448 takes the form of a cylindricalchamber in a Teflon orifice housing 473. A sapphire orifice plate 486defines an entrance or primary orifice to receive pressurized gas intothe chamber from conduit 460. A sapphire orifice plate 488 defines anexit or secondary orifice through which the merged liquid and gas leavethe chamber. In addition, a liquid receiving entrance 490 conducts thesample liquid into the chamber.

In one suitable version of nebulizer 450, primary orifice 486 has adiameter of 0.006 inches, and secondary orifice 488 has a diameter of0.008 inches. The chamber has a diameter of 0.020 inches, and an axiallength, i.e. space in between orifice plates 486 and 488, of 0.020inches.

More generally, the secondary orifice diameter is larger than theprimary orifice diameter, yet less than the diameter of the cylindricalchamber. As compared to prior devices in which there is no secondaryorifice and the chamber is simply open at the exit end, there is a backpressure due to the secondary orifice which increases the feed pressureto the merger zone and results in a higher kinetic energy mixing of theliquid and compressed gas. This advantageously results in smaller sampleliquid droplets in the aerosol leaving the merger zone.

As the size of the secondary orifice is reduced, the droplet size isreduced and the back pressure is increased. When the sample liquid iswater, it has been found satisfactory to form the secondary orifice andthe primary orifice at a diameter ratio of 2 to 1 as indicated by thediameters given above. For a sample liquid with a boiling point lowerthan water, the preferred diameter ratio is closer to 1, yet thesecondary orifice remains larger than the primary orifice.

The higher energy in the merger zone more effectively breaks up theliquid. The secondary orifice also appears to improve the efficiency ofthe impactor downstream. The ratios of primary and secondary orificediameters can be selected to vary the pressure at the liquid entrance tothe merger zone, relative to atmospheric pressure. Depending on thediameter ratio, air inlet pressure and liquid flow rate the liquidpressure can be adjusted from below atmospheric pressure to a pressurenearly equal to the inlet air pressure.

As seen in FIG. 10, impactor 480 is disposed coaxially with merger zone448, spaced apart in the axial direction from orifice plate 488. Theimpactor cooperates with housing section 472 to form a thin, somewhathemispherical path to accommodate the flow of air and droplets beyondthe merger zone. The smaller droplets tend to follow the air flow, whilethe larger droplets tend to collide with impactor 480 and are removedfrom the aerosol stream. Thus, the aerosol moving into conditioning zone462, upwardly and to the right as viewed in FIG. 8, includes only thosedroplets below a size threshold determined largely by the axial spacingbetween secondary orifice 488 and impactor 480. The size threshold isincreased by increasing the axial spacing, and reduced by moving theimpactor closer to orifice plate 488.

The droplets impinging upon impactor 480 may remain on the impactormomentarily, but eventually descend to reservoir 468 then drain from thenebulizer. If desired, impactor 480 may be formed of sintered metal toprovide a porous structure that more effectively prevents the larger,impacting droplets from interfering with the aerosol flow.

A secondary gas may be introduced into nebulizer 450 at a locationupstream of the nebulization region. The secondary gas sweeps dead spacein the nebulization region resulting in a faster response, reduced axialdiffusion, and less smearing of the output due to mixing.

As was discussed above in general, once the aerosol is formed, theliquid in the droplets must be evaporated before the droplets have achance to collide and coalesce. Drying can be accomplished usingdilution air, heated air or heating the liquid.

Once the liquid is evaporated, the particles in the aerosol can becounted and/or sized by a number of techniques including, but notlimited to Optical Particle Counters (OPCs), Condensation ParticleCounters (CPCs) and Scanning Mobility Particle Sizers (SMPS). OPCs aresimilar to those used in liquids. They size and count individualparticles as they pass through a laser beam. Examples of OPCs includethose made by Particle Measuring Systems, RION, Horiba, Particle SizingSystems, and Hach Ultra.

Referring to FIG. 11, a CPC 500 is capable of measuring very smallparticles in aerosols. They act as “particle size amplifiers” in frontof a single particle counting optical detector. Particles drawn into thesensor pass through appropriately cooled and heated sections of a wetwalled condenser. The differing mass and thermal diffusivities of themolecules of water vapor and air, create a supersaturated region inwhich the water vapor condenses on to the particles. The liquid dropletscontaining the particles grow to a few micrometers in diameter which arethen detected optically with very high signal-to-noise. CPCs areavailable that use a number of working fluids including butanol andwater. By varying the design conditions, they can have detection limitsvarying from about 1 to 20 nm. CPCs by themselves do not measureparticle size distributions. They simply determine the concentration ofparticles larger than a size determined by their operating conditions.However, several CPCs with different detection limits can be combined todetermine a size distribution. Alternately, a CPC can be combined withan SMPS to determine the size distribution.

In one embodiment, the apparatus of the present invention includes theNebulizer/Impactor 450 and a CPC 500 with a detection limit preferablybetween 20 and 30 nm. This embodiment is believed to be best suited forhigh purity liquid to measure concentration above a defined threshold,but without particle size distribution (PSD) measurement.

In another embodiment, the apparatus of the invention includes aNebulizer/Impactor 450 and a Scanning Mobility Particle Sizers (SMPS).This embodiment is believed to be best suited for determining PSD.

Although the apparatus and method of the invention has been described inconnection with the field of semiconductor device manufacture, it canreadily be appreciated that it is not limited solely to such field, andcan be used in other fields.

FIG. 12 is a graph of droplet size distributions (differentialconcentration vs. droplet diameter measured in um) produced by variouscombinations of nebulizers with impactors. Differential concentration ismeasured in d (#/cm³) per d log (D_(P)). The graph includes linesillustrating fits of the PSD to a log-normal distribution. The dropletsize distributions were measured by forming an aerosol from a sucrosesolution, drying the droplets, measuring the residue PSD and calculatingthe droplet PSD using the equations above. The graph shows thatCombination D has the best distribution in that it has the smallest andmost uniform droplets, and virtually no droplets are larger than 10 um.

FIG. 13 is a graph of the cumulative particle concentration over time(in hours) measured using the nebulizer-impactor combination D.Cumulative particle concentration is measured in #/ml. greater than orequal to 20 nm. The detector is a CPC with a 20 nm detection limit.Particle detection is in ultra purified water (UPW) containing 0.4 ppbnon-volatile residue (NVR). The graph shows a detection limit ofapproximately 8000/ml. The limit would be lower in water with lowerresidue content. FIGS. 14 and 15 are graphs of particle cumulativeconcentration versus time (in minutes) for detection of particles inultra pure water (UPW). FIG. 15 is for 22 nm silica particles and FIG.14 is for 30 nm polystyrene latex (PSL) particles. These graphs show thedetection response of combination D (20 nm) CPC to low concentration ofthe two particle types. FIG. 16 is a graph of differential residualconcentration (d(nm³/cm³)/d log (D_(p))) versus particle size (in nm)which shows the ability to size 30 nm particle PSL. One sizing wasconducted with a Combination D apparatus with an SMPS detector. Anotherwas conducted with a dynamic light scattering (DLS) instrument, moreparticularly with a NICOMP 380ZLS made by Particle Sizing Systems, SantaBarbara, Calif. The comparison shows generally good agreement. TheCombination D apparatus with SMPS permits measurement of actual numberconcentration. In contrast, DLS only provides relative concentrations.The Combination D apparatus also provides a more detailed measurement ofPSD. DLS on the other hand assumes that the particles are log-normallydistributed. FIG. 17 is a graph of differential number concentration vs.particle diameter which measures CMP slurry PSD using a Combination Dapparatus with an SMPS detector. The graph shows good separation betweenresidue and slurry particles. And FIG. 18 compares this method with DLS(using the NICOMP 380ZLS). The graph shows good agreement between thetwo processes. The differential number concentration determined via thismethod is normalized to a maximum concentration of 1 since DLS onlygives relative concentrations. FIG. 19 shows the change in slurry PSDover time during handling via a graph of differential numberconcentration versus particle diameter. Successive times 1-9 aregraphed. The number of smaller particle decreases over time while thenumber of large (i.e. greater than approximately 250 nm) increases. Thisindicates that particle agglomeration is occurring due to handling. Thisis an example of usefulness of the method of the invention. FIGS. 20 and21 provide measurements of filter retention. Filters were challengedwith polydisperse mixtures of PSL particles ranging from 20 to 500 nm indiameter. The graph (cumulative concentration in #/ml vs. particlediameter in nm) in FIG. 20 shows concentrations of particles upstream(feed) and downstream (filtrate) of the filter. The graph in FIG. 21(percentage retention vs. particle diameter (nm)) shows retention of thefilter as a function of particle size.

The embodiments above are chosen, described and illustrated so thatpersons skilled in the art will be able to understand the invention andthe manner and process of making and using it. The descriptions and theaccompanying drawings should be interpreted in the illustrative and notthe exhaustive or limited sense. The invention is not intended to belimited to the exact forms disclosed. While the application attempts todisclose all of the embodiments of the invention that are reasonablyforeseeable, there may be unforeseeable insubstantial modifications thatremain as equivalents. It should be understood by persons skilled in theart that there may be other embodiments than those disclosed which fallwithin the scope of the invention as defined by the claims. Where aclaim, if any, is expressed as a means or step for performing aspecified function it is intended that such claim be construed to coverthe corresponding structure, material, or acts described in thespecification and equivalents thereof, including both structuralequivalents and equivalent structures, material-based equivalents andequivalent materials, and act-based equivalents and equivalent acts.

1. A method for analyzing particles in a liquid, comprising the stepsof: a. forming an aerosol containing droplets from the liquid, b.isolating small, uniformly sized droplets from the droplets, c. removingliquid from the small, uniformly sized droplets by evaporating to apredetermined dryness, and d. analyzing residual particles by countingand/or sizing.
 2. The method of claim 1, wherein small, uniformly sizeddroplets are less than 10 um. in diameter.
 3. The method of claim 1,wherein the median diameter of small, uniformly sized droplets is lessthan 1 um, and the geometric standard deviation of the small, uniformlysized droplets is smaller than or equal to 1.4.
 4. The method of claim1, wherein the method measures particle concentrations in high purityliquids and wherein the liquid is a high purity liquid.
 5. The method ofclaim 1, wherein the method measures particle size distribution incolloidal suspensions and wherein the liquid and particles are in acolloidal suspension.
 6. The method of claim 1, wherein the method isused to measure particle retention of a filter.
 7. The method of claim1, wherein the step of forming an aerosol comprises the steps of: i.inputting the liquid to a mixing zone, ii. inputting pressurized gas tothe mixing zone; and iii. restricting output from the mixing zone togenerate a backpressure in opposition to input of the liquid and thepressurized gas, whereby the kinetic energy of mixing increases anddecreases the size of droplets leaving the mixing zone.
 8. The method ofclaim 1, wherein the step of isolating small, uniformly sized dropletsfrom the aerosol involves forming a thin, essentially hemispherical flowpath of the aerosol droplets whereby large droplets leave the flow pathand are impacted and removed.
 9. The method of claim 1 wherein the stepof removing liquid is accomplished by a process selected from the groupof processes consisting of heating the aerosol, applying a stream ofdilution air, applying a stream of heated air, heating the liquid, andfast evaporation.
 10. The method of claim 1, wherein the step ofanalyzing residual particles is accomplished by a process selected fromthe group of processes consisting of condensation particle counting andscanning mobility particle sizing.
 11. A method of analyzing particlesin a liquid, comprising the steps of: a. forming an aerosol containingdroplets from the liquid and isolating small, uniformly sized dropletsfrom the droplets by: i. inputting the liquid to a mixing zone, ii.inputting pressurized gas to the mixing zone, iii. restricting outputfrom the mixing zone to generate a backpressure in opposition to inputof the liquid and the pressurized gas, whereby the kinetic energy ofmixing increases and decreases the size of small droplets leaving themixing zone, and iv. forming a thin essentially hemispherical flow pathof droplets leaving the mixing zone whereby large droplets leave theflow path and are impacted and removed; b. removing liquid from thesmall, uniformly sized droplets by evaporating them to a predetermineddryness, and c. analyzing residual particles by counting and/or sizing.12. An analysis apparatus, comprising a droplet former for forming smalldroplets, an evaporator communicatively connected to the droplet former,and a detector communicatively connected to the evaporator for detectingparticles.
 13. The apparatus of claim 12, wherein small droplets areless than 10 um. in diameter.
 14. The apparatus of claim 13, wherein themedian diameter of droplets are less than 1 um., and the geometricstandard deviation of droplets is smaller than or equal to 1.4.
 15. Theapparatus of claim 12, wherein the apparatus measures particleconcentration in high purity liquids and wherein the liquid is a highpurity liquid.
 16. The apparatus of claim 12, wherein the apparatusmeasures particle concentration and size distribution in colloidalsuspensions and wherein the liquid and particles are in a colloidalsuspension.
 17. The apparatus of claim 12, wherein the apparatus is usedto measure particle retention of a filter.
 18. The apparatus of claim12, wherein the droplet former comprises a housing forming a mixingchamber having (i) a liquid entrance for receiving a sample liquid intothe chamber, (ii) a primary orifice having a first diameter forreceiving a pressurized gas into the chamber for merger with the sampleliquid to generate an aerosol composed of multiple droplets of thesample liquid suspended in the gas, and (iii) a secondary orifice havinga second diameter for conducting the aerosol out of the chamber.
 19. Thedevice of claim 18, wherein the second diameter is less than a majordimension of the mixing chamber taken in a direction substantiallyperpendicular to an axis of the secondary orifice so as to restrict flowout of the mixing chamber to generate a back pressure in opposition toentry of the sample liquid and the pressurized gas into the chamber. 20.The device of claim 19 wherein: the second diameter is less than onehalf of the major dimension of chamber.
 21. The device of claim 19wherein: the second diameter is larger than the first diameter.
 22. Thedevice of claim 19 wherein: the mixing chamber is cylindrical andcoaxial with the secondary orifice.
 23. The device of claim 22 wherein:the primary orifice is coaxial with the secondary orifice and thechamber.
 24. The device of claim 22 wherein: the chamber has an axiallength less than a diameter of the chamber and greater than the seconddiameter.
 25. The device of claim 19 further including an impactorcoaxial with the mixing chamber and spaced apart axially from thesecondary orifice downstream of the chamber, said impactor having aconvex upstream surface cooperating with a concave surface of thehousing to form a generally hemispherical path for conveying the aerosolaway from the chamber.
 26. The device of claim 25 wherein the impactoris movable axially with respect to the housing to selectively adjust theaxial spacing between the impactor and the secondary orifice.
 27. Theapparatus of claim 12 wherein the evaporator removes liquid and isselected from the group of elements consisting of a heater for heatingthe aerosol, an applicator applying a stream of dilution air, anapplicator for applying a stream of heated air, a liquid heater, and afast evaporator.
 28. The apparatus of claim 12, wherein the detectoranalyzes residual particles and is selected from the group of elementsconsisting of a condensation particle counter and a scanning mobilityparticle sizer.
 29. An analysis apparatus for measuring particleconcentration, including very low concentrations of very small particlesin high purity liquids, comprising: a. a nebulizer/impactor for formingdroplets and for isolating small, uniformly sized droplets therefrom,the nebulizer/impactor comprising a housing forming a mixing chamberhaving (i) a liquid entrance for receiving a sample liquid into thechamber, (ii) a primary orifice having a first diameter for receiving apressurized gas into the chamber for merger with the sample liquid togenerate an aerosol composed of multiple droplets of the sample liquidsuspended in the gas, and (iii) a secondary orifice having a seconddiameter for conducting the aerosol out of the chamber; and  an impactorcoaxial with the mixing chamber and spaced apart axially from thesecondary orifice downstream of the chamber, said impactor having aconvex upstream surface cooperating with a concave surface of thehousing to form a generally hemispherical path for conveying the aerosolaway from the chamber; b. an evaporator communicatively connected to thenebulizer/impactor for removing liquid from the droplets and generatingparticles, and c. an analyzer, communicatively connected to theevaporator, for counting or sizing reside particles.
 30. The apparatusof claim 29, wherein the analyzer is a condensation particle counter.31. An analysis apparatus for measuring particle concentration andparticle size distribution in colloidal suspensions and wherein theliquid and particles are in a colloidal suspension, comprising: a. anebulizer/impactor for forming droplets and for isolating small,uniformly sized droplets therefrom, the nebulizer/impactor comprising ahousing forming a mixing chamber having (i) a liquid entrance forreceiving a sample liquid into the chamber, (ii) a primary orificehaving a first diameter for receiving a pressurized gas into the chamberfor merger with the sample liquid to generate an aerosol composed ofmultiple droplets of the sample liquid suspended in the gas, and (iii) asecondary orifice having a second diameter for conducting the aerosolout of the chamber; and  an impactor coaxial with the mixing chamber andspaced apart axially from the secondary orifice downstream of thechamber, said impactor having a convex upstream surface cooperating witha concave surface of the housing to form a generally hemispherical pathfor conveying the aerosol away from the chamber; b. an evaporatorcommunicatively connected to the nebulizer/impactor for removing liquidfrom the droplets and generating particles, and c. an analyzer,communicatively connected to the evaporator, for counting or sizingreside particles.
 32. The apparatus of claim 31, wherein the analyzer isa scanning mobility particle sizer.
 33. An analysis apparatus formeasuring particle retention for a filter, comprising: a. anebulizer/impactor for forming droplets and for isolating small,uniformly sized droplets therefrom, the nebulizer/impactor comprising ahousing forming a mixing chamber having (i) a liquid entrance forreceiving a sample liquid into the chamber, (ii) a primary orificehaving a first diameter for receiving a pressurized gas into the chamberfor merger with the sample liquid to generate an aerosol composed ofmultiple droplets of the sample liquid suspended in the gas, and (iii) asecondary orifice having a second diameter for conducting the aerosolout of the chamber; and  an impactor coaxial with the mixing chamber andspaced apart axially from the secondary orifice downstream of thechamber, said impactor having a convex upstream surface cooperating witha concave surface of the housing to form a generally hemispherical pathfor conveying the aerosol away from the chamber; b. an evaporatorcommunicatively connected to the nebulizer/impactor for removing liquidfrom the droplets and generating particles, and c. an analyzer,communicatively connected to the evaporator, for counting or sizingreside particles.
 34. The apparatus of claim 33, wherein the analyzer isa scanning mobility particle sizer.